(1) Field of the Invention
The present invention relates to a semiconductor stacked CMOS device adapted to enhance the insulative proof-voltage of a gate insulation film in a MOS FET and to have a high current drive capability, and also relates to a method of producing thereof.
(2) Description of the Related Art
A conventional stacked CMOS device is, for example, disclosed in the paper "Process and Performance Comparison of an 8k bit SRAM in Three Stacked CMOS Technologies," IEEE ELECTRON DEVICE LETTERS, Vol. 6, No. 10, October, 1985, Pages 548 to 550.
Conventionally, in this kind of CMOS devices, P-type and N-type diffusion layers are formed in a plane on a semiconductor substrate and then an N-channel MOS FET and a P-channel MOS FET are formed, respectively.
However, in recent years, a method has been proposed in which an N-channel MOS FET with a polycrystalline silicon gate electrode is formed, and then a polycrystalline silicon ("Si") layer which acts as a channel area of a P-channel MOS FET is formed through an insulation film.
FIGS. 4(A) and 4(B) show an example of the conventional stacked CMOS structure. In the process of producing this stacked CMOS structure, the gate insulation film of an upper P-channel MOS FET can be obtained by oxidizing the upper surface of a polycrystalline Si gate electrode.
That is, as shown in FIG. 4(A), the upper surface of the polycrystalline Si gate electrode 22 of an N-channel MOS FET on an N-channel in a substrate 21 is oxidized to form a gate insulation film 23, and thereafter, a second polycrystalline Si channel area 24 of a P-channel MOS FET is formed. Then, the side wall structure is removed by anisotropic etching, and a doped oxide film 25 is formed by spin coating.
Next, after a protective oxide film 26 is formed, it is heat-treated, whereby the boron ions doped in the oxide film 25 are diffused in the second polycrystalline Si 24 to form P.sup.+ source and drain regions of the P-channel MOS FET.
However, in the above-mentioned conventional stacked CMOS structure, since the polycrystalline Si gate insulation film 23 on the gate electrode 22 contains impurities in a high concentration, the impurities are diffused into the gate insulation film 23, resulting in a detrimental effect upon the insulative proof-voltage and the like.
Further, a surface-roughness depending upon a crystal orientation existing on the surface of the polycrystalline Si also affects the insulative proof-voltage.
Further, there is a problem that since the upper MOS FET is made of polycrystalline Si, a high current drive capability cannot be obtained.
One object of the present invention is to provide a semiconductor stacked CMOS device which includes a gate insulation film immune against an insulation proof-voltage and an upper layer MOS FET with high current drive capability.
Further, another object of the present invention is to provide a method of producing a semiconductor stacked CMOS device which can solve the problem that the insulative proof voltage of polycrystalline Si is lowered by surface roughness depending upon a crystal orientation.